This invention relates to a method and apparatus for heating deionized water or other processing liquids to a desired high temperature and for maintaining a flow of such heated liquid at desired commercial rates. There has long been a need for a reliable heater for heating deionzed water and other processing liquids to fairly high temperatures such as 90 degrees Celsius and for providing a reliable constant flow of such water or liquid particularly for use in semi-conductor manufacturing and also for certain other bio-medical processes and cleaning applications. There are and have been a number of heating devices for deionized water, but those presently available have shortcomings or limitations of one kind of another. Because deionized water is such an effective solvent, it wants to dissolve or liberate ions from almost any metallic object that it comes in contact with. Consequently, there are only a limited number of materials commonly in use for containing and/or transporting deionized water, particularly, hot deionized water. Such hot deionized water is used in semi-conductor manufacturing in cleaning the usual semi-conductor substrate material, such as silicon and removing undesired particles therefrom. In order to contain hot deionized water, it is necessary to use very inactive materials such as polytetrafluoroethylene (PTFE Teflon) or, PFA Teflon (Perfluoroalkoxy) Teflon or PVDF (polyvinylidene fluoride) all of which materials are useful up to temperatures somewhat below their melting points. For substantially higher temperature applications, such materials as titanium or gold plated 214 quartz or gold plated polysulfone, PTFE, synthetic sapphire, or polysilicon may be used. The particular choice of materials, as will be appreciated by those skilled in the art, depends upon the temperatures generated in a particular heated design and also as to the particular processing liquid which is being heated. While it is common to use Teflon to contain deionized water it has limited ability to withstand heat. It has been a practice to heat deionized water by means of Teflon coated heater wires. These are susceptable to burning out because of overheating the Teflon, thus exposing the heating wires which contaminates the water. Another system which has been used involves the use of an immersion heater in a tank, heating water through which run a number of Teflon tubes carrying the deionized water, thus the heat from the tank passes through the walls of the Teflon tubes the arrangement operating like a heat exchanger. But this last system is limited in the temperature it will accommodate because of the Teflon, which, of course, limits the rate of heat transfer to the deionized water.
Water, particularly warm water, is a favorable growth medium for many kinds of organisms such as bacteria and fungi. A heater involving a reservoir of any substantial size wherein the heated deionized water is caused or permitted to stand for any significant time is highly susceptible to the undesirable generation of such organisms.
From the foregoing it will be appreciated that there is a need for a heater for deionized water and other processing liquids which will heat the water quickly to a temperature such as 90 degrees Celsius which can continue to supply deionized water at such temperature in commercial quanitites without significant degradation over a substantial range of flows.